Revert "Manually written Transformable instance for SegTree"

This reverts commit 34b8929083da850f6060370853e89d1ca7c5eeb3.

In the end 7.8.1 actually supports this derived instance, without the
need for a manual instance due to role restrictions.

Conflicts:
	src/Diagrams/Trail.hs
Merge branch 'master' into roles